I'm not an expert on these, but I have a few hypotheses.
The reefer unit is either pumping less heat out while driving, or the box is allowing more heat in. I'd start by determining if the freezer plate shows signs of not freezing anymore; the pattern of ice melting on it would be different if less heat is being pumped out (ice thaws from plate outward, across most or all of plate, ice comes loose in sheets) or more heat getting into box (edges and fringes of plate thawing, but a goodly patch of ice still frozen solid to plate). This is the first branch of the diagnostic tree.
If the reefer unit is chilling less, perhaps the chimney is causing way too much draft while moving. This could cause the chiller to absorb less heat from the flame, as well as possibly sink away heat produced by electric power, thus providing less chilling while driving.
OTOH, if the reefer still pumps the same amount of heat but more heat gets in, perhaps there is missing/damaged insulation that allows heated air from around the hot parts to blow against the outside of the box. An infared thermometer or thermal imaging device would help here, see if there are warm areas while driving. Many modern cameras have a night vision feature that is sensitive to IR. Also is there a light that somehow stays on when the engine is running?
It seems I love the mountains and deserts more than my friends do. I sure miss them!
1971 Streamline Imperial project "Silver Snausage", 1985 Coleman tent trailer, 1964 Little Dipper, 1975 Northwest "Proto Toyhauler", 2004 Harbor Freight folding, still seeking my Airstream.